Particle-in-cell simulation of a radioactive potential probe in wind

Summary form only given. Experiments have shown that the floating potential of a polonium 210 radioactive potential probe in air is a function of wind speed and direction. Polonium 210 decays by emitting 5.3-MeV alpha particles which have a range in STP air of about 3.8 cm. Each alpha particle creates about 10⁵ electron-ion pairs in its wake. Within a microsecond of their creation, the free electrons either recombine with positive ions or attach to molecules to create negative ions. The floating potential of the probe becomes more negative (≃-1.0 V) as the wind speed increases within tie range 0-5/ms. The floating potential is also observed to be more negative when alpha particles are emitted upstream rather than downstream. This phenomenon is being studied with a two-dimensional particle-in-cell simulation. Finite difference methods are used to solve Poissons equation and to move simulation particles under the influence of mobility, convection, diffusion, and recombination. A variable cell size has been used to provide improved resolution near the probe. Simulation particles represent positive and negative ions created by the emitted alpha particles. The number of simulation particles created per time step is determined by the activity (≃5 μCi) of the radioactive probe and by the length of the time step. Space charge separation and an electric field result from incomplete neutralization of negative charge on the radioactive probe